Method for manufacturing thin film oxide superconductors and superconductor devices by x-ray irradiation

ABSTRACT

A manufacturing method for the thin film superconductor is disclosed in which photons having energies larger than ultraviolet rays are irradiated to the thin film superconductor on or after formation of the thin film. Further, manufacturing methods for superconductive magnetic memory, Josephson device and superconductive transistor are disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconductor thin film and amanufacturing method therefor, and a thin film superconductor device anda manufacturing method therefor.

2. Prior Art

The P-type oxide superconductive material using electron holes as theelectric charge transfer carrier in the normal conductive state ofLa-Sr-Cu-O base, Y-Ba-Cu-O base, Bi-Sr-Ca-Cu-O base, and Tl-Ba-Sr-Cu-Obase materials, etc., has not been clarified in detail as to thesuperconductivity mechanism thereof, but the transition temperaturethereof is higher than the temperature of liquid nitrogen. Therefore, anapplication to various fields of electronics such as quantuminterference elements.

These materials show the change from an insulator (semi-conductor) to asuperconductor, depending upon the amount of oxygen atoms contained inthe crystal, that is, the state of oxidization. In order to obtain agood superconductor material, the improvement of crystallinity andcontrol of oxidization state are required. According to themanufacturing method for the sintered superconductor material obtainedso far, a high temperature process above 800° C. in the oxygen ambientand a slow cooling process below 100° C. are required, whichnecessitates an installation of a high temperature furnace and a longprocessing time.

Unlike these P-type superconductor materials, materials of such asNd-Ce-Cu-O base, Nd-Cu-o-F base, etc. have crystalline structure of Nd₂CuO₄ type and N-type oxide superconductor materials including electronsas electric charge carriers in the normal conductivity state.

The N-type superconductor material like the P-type superconductormaterial shows the change from an insulator (semiconductor) to asuperconductor, depending upon the amount of oxygen deficiency containedin the crystal, that is, the state of oxidization. In order to obtain agood superconductor material, improvement of crystallinity and controlof oxygen deficiency (state of oxidization) are required. According tothe manufacturing method for the superconductor material so farobtained, a careful annealing process at a high temperature above 800°C. under the vacuum (reduction atmosphere) was necessary.

These new superconductor materials can be prepared only by the sinteringprocess primarily at the current technical level and may be availableonly in the form of a ceramic powder or block. On the other hand, incase of placing these materials into practical use, although processinginto thin films is strongly demanded, it is very difficult to prepare athin film of good superconductive characteristics according to theconventional technique.

In each of P-type and N-type superconductor thin films so far prepared,the current density in the superconductivity critical condition is smalland the time-dependent logarithmic decrease of current density isremarkable, and therefore, it has been considered difficult to realize asuperconductor device of high stability and reliability.

In the case of using these p-type and N-type cuprocompoundsuperconductors as a superconductive magnetic memory, various methodshave been devised to distinguish 1 and 0. In the case of utilizingsuperconductor thin film for super conductive magentic memory, accordingto the conventional proposal, the distinction between 1 and 0 memorystates are made by detecting fluxoids replenished in the superconductormaterial or in the space enclosed by superconductors with a detector toexamine the presence or non-presence of fluxoids, and by using acuprocompound superconductor of high transition temperature, it ispossible to operate a superconductive magnetic memory at the liquidnitrogen temperature.

Conventionally, there have been proposed various superconductivemagnetic memories using niobic nitride (NbN) of an A15 typetwo-component compound or niobic germanium (Nb₃ Ge). However, thesuperconductivity transition temperatures T_(c) of these materials are24 K. at the most. Furthermore, as perovskite compounds, there are knownmaterials of Ba-Pb-Bi-O base (Japanese Patent Laid-Open Publication No.SHO60-173885/1985) and there are researched a lot of superconductorelements using materials of this base. However, T_(c) of this materialis relatively as low as 13 K. and it is difficult to place this materialinto practical use. The high temperature superconductor material has atransition temperature higher than the liquid nitrogen temperature andit is greatly expected to place said material into practical use.

Recently, there is reported a field effect type superconductortransistor using high temperature superconductor thin film (A. Yoshida,H. Tamura, N. Fujimaki, and S. Hasuo, Extended Abstracts of 1989International Superconductivity Electronics Conference, DE2-2). Thissuperconductor transistor is prepared in the following manner: That is,after forming Ba-Y-Cu-O superconductor thin film on a MgO substrate, byheat-treating at 600° C. in the nitrogen atmosphere, said superconductorthin film is reduced to prepare a thin film of semiconductivecharacteristics deficient of oxygen, and after a gate insulating filmmade of BaF₂ and a gold gate electrode are formed on said semiconductorthin film, by exposing them to oxygen plasma, oxygen is doped so as torecover superconductive characteristic in the thin film portionscorresponding to the source area and drain area, thus to prepare asuperconductor transistor.

PROBLEM TO BE SOLVED BY THE INVENTION

Forming superconductor into thin film according to the present inventionis accomplished by decomposing superconductor raw material into minutecorpuscles of atomic state and then depositing the same on a substrateas a compound thin film. As compared with a sintered body, a film ofgood crystallinity and of better homogeneity can be obtained. However,the crystallinity of thin film obtained in the formation process ofcomplex compound thin film and the amount of oxygen (deficiency) takentherein are not always sufficient to obtain good superconductivecharacteristics, and to provide the best superconductivecharacteristics, an improvement of crystallinity and sufficient intakeof oxygen or processing for generating oxygen deficiency are required.

Generally, the flux pinning force of a cuprocompound superconductor thinfilm is very weak, and a large flux creep is observed at the liquidnitrogen temperature: Because of the existence of this phenomenon, inthe case of using the cuprate compound superconductor thin film as amemory medium of the superconductive magnetic memory, the instabilitysuch as time-dependent changes in the characteristics thereofconstitutes a serious problem for placing the transistor into practicaluse. In order to improve the critical current density of the cupratecompound superconductor thin film and increase the activation energy forthe flux creep, it is necessary to generate appropriate pinning centersfor fluxoid quantums in the thin film at a proper density. The presentinvention aims to solve the above-described conventional problem and toprovide a superconductor thin film having a large critical currentdensity and a strong flux pinning force and a superconductive magneticmemory of excellent memory characteristic using the same.

According to the conventional method of heating the high temperaturesuperconductor at a high temperature in the vacuum, the integration ofthe superconductor thin film with the normal semiconductor circuit isdifficult, and the method of irradiating reducing active ions such ashydrogen ions has disadvantages in that the superconductor is damaged byactive ions and the superconductor characteristics are not recoveredeven by an oxidization processing effected thereafter, and therefore,application to the Josephson devices is difficult. The method ofirradiating photons having energy larger than ultraviolet rays accordingto the present invention does not have such a problem and provides amanufacturing method for Josephson devices using new high temperaturesuperconductor thin films.

Furthermore, in the superconductor transistor prepared by usingconventional Ba-Y-Cu-O base superconductor thin film and making thesource area and the drain area superconductive through control of oxygendensity, there is a problem that due to diffusion of oxygen atoms,time-dependent changes in characteristics of the device are large, andthe realization of a superconductor transistor of small time-dependenceis strongly desired.

SUMMARY OF THE INVENTION

One of objects of the present invention is to provide superconductorthin films having a large critical current density and a small giantflux creep (large time dependence of magnetization).

Another object of the present invention is to provide a manufacturingmethod for superconductor thin films which is capable of controllingdefects such as oxygen defects in the thin films.

A further object of the present invention is to provide superconductortransistors having a small flux creep.

A still further object of the present invention is to provide amanufacturing method for superconductor transistors.

One of the other objects of the present invention is to providesuperconductor magnetic memories having a small giant flux creep and amanufacturing method therefor.

One more object of the present invention is to provide a method formanufacturing Josephson devices effectively.

In the superconductor thin film according to the present invention aswell, the crystalline structure thereof and the content of oxygen atomsor oxygen deficiencies, that is, the state of oxidization change,depending on the atmosphere and the temperature, and the superconductivecharacteristics are also changed. Although said superconductivecharacteristics can be improved to some extent by appropriateimprovements in the crystalline structure and oxidizing or reducingprocessing, the effect is affected by the diffusion behavior of oxygen,or is not effective to portions once exposed to the outside air andsubjected to an irreversible oxidizing or reducing reaction.

The inventors of the present invention have found out that excellentsuperconductor thin films can be realized with a good controlability andstability by carrying out irradiation of photons having energy (morethan 2 eV and less than 100 k eV) larger than ultraviolet rays and anoxidization processing as the processing for crystallinity improvementof superconductor thin films and for control of the content of oxygendeficiencies upon formation of said complex compound superconductor thinfilms or thereafter, and have developed the present invention. Theoxygen deficiencies of the high temperature superconductor thin film areof smaller free energy as compared with the surrounding thereof andthereby, form effective pinning centers for the fluxoid quantums in thecritical state. According to the aforementioned irradiation of photonshaving an energy larger than ultraviolet rays, the oxygen deficienciescan be introduced without breaking the ordering of the metal elements.Since the coherence length of the high temperature superconductor isshort, it is effective for pinning of fluxoid quantums to uniformlygenerate oxygen deficiencies in the crystal. In the superconductor thinfilm of less than several micrometers thickness, depending on the energyof photons used, it becomes possible to generate homogeneous oxygendeficiencies throughout the entire film thickness. Although the criticalcurrent density of the thin film superconductor can be improved and theactivation energy of the flux creep can be increased even by this only,it has been found that when photons are irradiated onto the thin filmsurface at an incident angle below the total reflection angle, oxygendeficiencies are formed in the surface first layer only. Such asuperconductor thin film subjected to irradiation of photons havingenergy larger than ultraviolet rays and, if necessary, to an oxidizationprocessing has a magnetization curve completely different from thoseconventionally known, and since it is small in the time-dependent changeand stable, it has an excellent potential for a superconductive magneticmemory medium.

The manufacturing method for a Josephson device according to the presentinvention is characterized by:

a step of carrying out an oxidization processing selectively only on thetwo areas separated by a minute gap portion, after irradiating photonshaving an energy larger than ultraviolet rays onto the surface of thesuperconductor thin film; or

a step of carrying out an oxidization processing on the surface ofmetallic oxide superconductor thin film simultaneously on or afterirradiating photons having an energy larger than ultraviolet raysselectively only onto the two portions separated by a minute gap portionon the surface of metallic oxide superconductor thin film; or

a step of performing an oxidization processing only on the two areasseparated by a minute gap portion on the superconductor thin filmsurface simultaneously on or after irradiating photons having an energylarger than ultraviolet rays selectively only onto said two areas; inorder to form a junction portion on said gap portion of saidsuperconductor thin film.

Furthermore, as a mask pattern for forming the junction portion, anelectron beam resist or a negative resist for opto-lithography of anacrylic resin such as PMMA or of a styrene resin such as CMS can beused. Furthermore, it is possible to irradiate oxygen ions orquasistable oxygen atoms in the excited state, or to perform anoxidization process in an atmosphere containing ozone.

In the superconductor transistor according to the present inventionwherein a gate electrode is formed on the surface of the channel layerthrough a gate insulating film, a superconductive source area, saidchannel layer and a superconductive drain area are made of hightemperature superconductor thin films, and can be realized by using, forsaid superconductive source and drain areas, materials of improvedcrystallinity over said channel layer by the irradiation of photonshaving an energy larger than ultraviolet rays. Furthermore, themanufacturing method for the superconductor transistor comprises thefollowing steps:

a step of forming by patterning respective portions corresponding to thesuperconductive source area, the channel layer and the superconductivedrain area, after forming the superconductor thin film;

a step of forming a gate insulating film and a gate electrode on saidthin film portion corresponding to said channel layer; and

a step of forming the superconductive source, superconductive drain andchannel layer by performing an oxidization processing simultaneously onor after irradiating photons having energy larger than ultraviolet raysonto said thin film from the side of said gate electrode.

Particularly for the gate electrode material, the use of metal orsilicide is preferably used to shield photons of a wavelength less thanthose of ultraviolet rays.

The thin film superconductor according to the present invention ishomogeneous as compared with conventional sintered bodies, and theimprovement of crystallinity and the content amount control of oxygenatoms or oxygen deficiencies are possible by the irradiation of photonshaving an energy larger than ultraviolet rays and heat treatment in anoxygen atmosphere, or the irradiation of oxygen ions or oxygen atoms inthe excited state. In the present invention, as a method for enhancingthe crystallinity of P-type superconductors, x-ray irradiation orultraviolet ray irradiation is performed simultaneously on the thin filmdeposition or suspending said deposition, as a method for theoxidization processing, the heat treatment in the oxygen atmosphere andirradiation of oxygen ions or oxygen atoms of excited state is performedconcurrently on the thin film deposition or suspending said deposition.In other words, the present invention is characterized in that byalternately repeating the thin film deposition process, thecrystallinity improvement processing and the oxidization processing, thesuperconductor material of a high performance is realized with excellentcontrollability and stability.

In the n-type superconductor, the crystallinity improvement and thecontrol of oxygen content are also possible by the irradiation ofphotons having energies larger than ultraviolet rays.

In either of the P-type and N-type superconductor thin films, theirradiation of photons having energies larger than ultraviolet raysreduces oxides by neutralizing negative ions such as O or F in thecrystal through the electron excitation and causing them to release fromthe crystal. Furthermore, since their potential energies are released inthe crystal lattice at the same time, a spike-type lattice vibration dueto relaxation of the excited state of inner kernels is excited. Thispromotes rearrangement of the atomic arrangement of positive ionsresulting in the enhancement of the orientation of the crystal.

When photons are irradiated on the thin film surface at an incidentangle below the total reflection angle, the oxygen deficiencies aregenerated in the first layer of the thin film surface without breakingthe ordering of metal elements. The oxygen deficiencies in thesuperconductor become effective pinning centers for fluxoid quantums,and are able to improve the critical current density and at the sametime to increase the activation energy of flux creep. By alternatelyrepeating the thin film deposition process, and the process ofirradiating photons having energies larger than ultraviolet rays at anincident angle below the total reflection angle, it becomes possible tocontrol the distribution of the surface oxygen deficiencies in theatomic scale. The fluxoid quantum lines do not pierce the film thicknessin straight lines but meander in the film thickness. In order toeffectively pin fluxoid quantum lines, it is preferable to distributeoxygen deficiencies at a distance from several to several hundredsangstroms in the film thickness direction. Since the flux pinning forcethereof is homogeneously strengthened in said superconductor thin film,it is possible to realize a superconductive magnetic memory of highperformance thereby.

Because the absolute value of magnetization differs greatly between astrong superconductor and a weak superconductor, it is possible todistinguish memory states by setting an appropriate threshold. In astrong superconductor thin film, since the flux pinning force ishomogeneously distributed, when the applied magnetic field is increased,the transition from a greatly demagnetized state to a slightlymagnetized state takes place abruptly at a certain magnetic field. Thistransition is irreversible, and once transferred to a slightlymagnetized state, it does not return to the greatly magnetized state. Itis also possible to constitute a superconductive magnetic memory byutilizing this irreversible transition and to distinguish 1 and 0 of thememory states.

Since the manufacturing method for the Josephson device according to thepresent invention requires only the irradiation of photons havingenergies larger than ultraviolet rays and the oxidization processing, ascompared with the conventional method of preparing a minute bridgestructure by etching, a Josephson device can be manufactured in a verysimple process. Furthermore, since any etching which may causedeformation is not used in this method, this method is effective forintegrating various devices. In the manufacturing method according tothe present invention, as a result that cuprooxide in the superconductoris reduced by the irradiation of photons having energies larger thanultraviolet rays onto the superconductor thin film surface, thesuperconductor characteristic thereof is deteriorated at first.Thereafter, only the two areas separated by a minute gap portion areselectively subjected to oxidization processing so as to restore thesuperconductivity reversibly, and at said gap portion, a Josephsonjunction is formed as a weak coupling. According to this method, sincethe oxidization processing is applied only to limited areas, circuitsdamaged by oxidization can be formed on the same substrate. Sincephotons are irradiated all over the surface, even a light source notcollimated can be used. Furthermore, by irradiating photons havingenergies larger than ultraviolet rays only onto the two areas separatedby a minute gap portion and thereafter performing oxidization processingonly, the superconductive characteristic of the irradiated areas areimproved, and said gap portion becomes a weak-coupled type Josephsonjunction. According to this method, the conventional opto-lithographymethod may be used as it is to selectively irradiate photons on thesuperconductor thin film surface and the oxidization processing need notbe performed partially, many of the techniques conventionally used formanufacturing semiconductor integrated circuits can be used as they are,and therefore the manufacture is easy. Furthermore, according to themanufacturing method of the present invention, photons having energieslarger than ultraviolet rays are irradiated only onto the two areasseparated by a minute gap portion on the superconductor thin filmsurface and thereafter only said two areas are subjected to theoxidization processing for improvement of superconductivecharacteristic, and on said gap portion, a weak-coupled Josephsonjunction is formed. According to this method, since the photonirradiation and oxidization processing are performed on the limitedareas of the metal oxide superconductor thin film surface, the portionsrequiring no formation of Josephson junctions are not affected by saidirradiation and oxidization processing, and therefore, it is effectivefor integration of Josephson devices and other functional devices.

In a preferred embodiment of the present invention, the superconductivesource area and the superconductive drain area are of high temperaturesuperconductor thin film of the same elements and the same compositionand by using material improved in the crystallinity by irradiation ofphotons for said superconductive source and drain areas only, excellentjunctions can be obtained between the source and the channel, andbetween the drain and the channel, and the time-dependent change in thecharacteristics of the device is reduced, and a field effect typesuperconductor transistor being stable and of high performance can berealized.

In a preferred embodiment of the manufacturing method for thesuperconductor transistor according to the present invention, byirradiating photons having energies larger than ultraviolet rays ontothe superconductor thin film from the gate electrode side, and byperforming oxidization processing simultaneously thereon or thereafter,only the source and drain areas are improved in crystallinity so as toform areas of superconductive characteristics better than the channellayer. At this time, since the gate electrode becomes the mask forphotons and the alignment of channel layer and the gate electrode iseffected self-aligningly, it becomes easy to manufacture even asuperconductor transistor of extremely short wavelength of about 0.1 μm.Particularly, as the material for the gate electrode, it is greatlyeffective to use metals of good light shielding property or silicide.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiment thereof with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic sectional view showing an apparatus formanufacturing N-type thin film superconductors according to a preferredembodiment of the present invention;

FIG. 2 is a drawing showing the temperature dependency of resistivityshowing the superconductive characteristic of a P-type thin filmsuperconductor based on the crystallinity improvement processing and theoxidization processing according to the present invention;

FIGS. 3(a), 3(b) and 3(c) are X-ray diffraction figures showing thecrystallinity of a P-type thin film superconductor based on thecrystallinity improvement processing and the oxidization processingaccording to the present invention;

FIG. 4 is a graph showing the temperature dependency of resistivityshowing the superconductive characteristic of a P-type thin filmsuperconductor based on the combination of various crystallinityimprovement processings and oxidization processings;

FIG. 5 is a schematic sectional view showing the basic constitution ofthe manufacturing device of N-type thin film superconductor according toa preferred embodiment of the present invention;

FIG. 6 is a graph showing the temperature dependency of resistivityshowing the superconductive characteristic of an N-type thin filmsuperconductor based on the crystallinity improvement processing and thereduction processing according to the present invention;

FIG. 7 is a X-ray diffraction figure showing the crystallinity of anN-type thin film superconductor based on the crystallinity improvementprocessing and the oxidization processing according to the presentinvention;

FIG. 8 is a schematic sectional view showing the basic constitution ofthe thin film superconductor characterized by the photon irradiation atan incident angle below the total reflection angle according to thepresent invention;

FIG. 9 is a sectional view of the superconductor thin film includingoxygen deficiencies periodically in the film thickness direction;

FIG. 10(a) is a graph showing the magnetic field change of thediamagnetization of the thin film superconductor in the cases where theprocessing according to the present invention was conducted and wheresaid processing was not conducted;

FIG. 10(b) is a pattern example of the superconductive magnetic memory;

FIG. 11 is a schematical sectional view showing the basic constitutionof the manufacturing device for superconductor thin film according to apreferred embodiment of the present invention;

FIG. 12 is a schematic perspective view for showing a manufacturingmethod of Josephson devices according to a preferred embodiment of thepresent invention;

FIG. 13 is a graph showing the temperature change of resistivity ofsuperconductor thin film after irradiation of photons having energylarger than ultraviolet ray immediately after preparation thereof andafter the oxidization processing;

FIG. 14 is a schematic perspective view for showing one of process amongthe manufacturing methods for Josephson devices according to a preferredembodiment of the present invention;

FIG. 15 is a sectional view showing the basic constitution of thesuperconductive transistor according to a preferred embodiment of thepresent invention; and

FIG. 16 shows schematical sectional views showing a manufacturing methodof superconductive transistors according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred Embodiment 1

A preferred embodiment of the present invention will be described withreference to the accompanying drawings.

As for the device constitution, as shown, for example, in FIG. 1, aformation tank wherein a vapor deposition source for thin filmdeposition, device for crystallinity improvement and reductionprocessing, and, for P-type superconductor, a device for oxidizationprocessing are provided is required. In the example shown in FIG. 1,there are used a high frequency magnetron sputtering device as a vapordeposition source, a X-ray source as a device for crystallinityimprovement, and a ECR oxygen plasma generating device for oxidizationprocessing of P-type superconductors. Depending upon the type ofcombination for the vapor deposition source, the crystallinityimprovement device and the oxidization processing device, it isnecessary to separate the formation tank and provide a load lockmechanism so as to move the substrate together with the holder thereof.

As shown in FIG. 1, a four-element compound thin film 12 is formed, forexample, by the sputtering method. In this case, as the substrate, asingle crystal substrate is effective for forming a 4-element compoundthin film of good crystallinity, and single crystals of magnesium oxide,LaAlO₃, LaGaO₃, strontium titanic acid, etc. are used.

The inventor of the present invention et al. have confirmed that in caseof attaching thin film 12 onto the rear face of substrate 11, in orderto provide superconductive characteristic, the temperature range of 500°to 900° C., and that of 400° to 1100° C. are suitable respectively forP-type and N-type. The optimum substrate temperature for optimizing thecrystallinity, composition and surface condition of compound thin filmfalls in this temperature range.

P-type superconductors A-B-Cu-O of Bi or Y base have not yet definitelydetermined about the crystal structure and composition formula, but isis considered that the smaller the oxygen deficiency amount, the higherthe superconductivity transition temperature. In sintered bodies, thestabilization of crystal and oxygen supply (oxidization processing) areperformed by a high temperature treatment of above 800° C. in oxygenatmosphere followed by a slow cooling process below 100° C./hr.

The inventors of the present invention have confirmed that thin films ofhigh crystallinity are excellent in the initial characteristics andstability on the long time basis, and control of the crystallinityduring film formation or immediately thereafter and the content amountof oxygen atoms or oxygen deficiency provides better superconductivecharacteristics. From the standpoint of the manufacturing process, whenfilms are once taken out of the thin film formation tank, moisture inthe air is adsorbed on the surface and reacts with the constituentelements of the thin film, resulting in deterioration of thecharacteristics. Therefore, it is necessary to incorporate thecrystallinity improvement and reduction processing, and the oxidizationprocessing for P-type superconductor into the thin film formationprocess as a part thereof.

The inventors of the present invention tried the following three typesof methods as the methods for the crystallinity improvement andreduction processing, and the processing for oxidizing P-typesuperconductor: 1) method of performing as the after-treatmentimmediately after thin film deposition, 2) method of performingsimultaneously with thin film deposition, and 3) method of performing byinterrupting thin film deposition, that is, method of alternatelyrepeating thin film deposition process, crystallinity improvement andreduction processing process, and oxidization processing process for aP-type superconductor, and confirmed that each method is able to obtainsuperconductor thin film of better characteristics as compared with thecase where no processing for crystallinity improvement or foroxidization is performed.

First, the inventors of the present invention found out that when duringformation of p-type superconductor thin film, X-ray irradiation is made,immediately after thin film formation, oxygen gas is introduced into theformation tank, and an oxidization processing as the after-treatment isperformed by the slow cooling process the same as for sintered bodies,an excellent superconductive characteristics is obtained. The inventorsfurther discovered that it is within a limited temperature range belowthe substrate temperature for thin film formation and above the normaltemperature that the crystallinity of thin film is improved by X-rayirradiation and oxygen is taken into thin film most effectively, andthat the processing for crystallinity improvement and reduction, and theoxidization processing for p-type superconductor can be performed mostefficiently and easily by performing the same within this temperaturerange for a given time. The inventors of the present invention furtherdiscovered that this effect is also observed when the processing forcrystallinity improvement and reduction, and oxidization processing forP-type superconductor are performed by interrupting the thin filmdeposition, that is, the thin film deposition process, the crystallinityimprovement and reduction process and the oxidization process for ap-type superconductor are alternately repeated. Since the temperaturefor performing such crystallinity improvement and reduction processing,and P-type superconductor oxidization processing differs depending uponthe kind of constituent elements of thin-film and the surface conditionthereof, it is necessary to select a proper temperature for each case.The inventors of the present invention confirmed that it is within atemperature range below 450° C. and above 300° C. With respect to theprocessing time, a necessary minimum value exists depending on the kindof thin film, film thickness and the surface condition.

The inventors of the present invention discovered that as a method forthe oxidization processing used in the case of performing thecrystallinity improvement and reduction processing and the oxidizationprocessing for p-type superconductor simultaneously with the thin filmdeposition or performing the same by interrupting the thin filmdeposition, that is, alternately repeating the thin film depositionprocess, the crystallinity improvement and reduction processing processand the oxidization processing process for P-type superconductor, t iseffective and simple to process by oxygen ions generated by thedischarge of a gas containing at least oxygen or to irradiate neutraloxygen atoms in the excited state. It was discovered by the inventorsthat high energy oxygen ions and neutral oxygen atoms effectivelyoxidize P-type superconductor thin film by using, for example, the ECRoxygen plasma generating device shown in FIG. 1, introducing oxygen gasor a mixture gas containing oxygen into the plasma generation chamber 16connected to said formation tank, generating discharge plasma byirradiating microwaves onto this gas, applying a magnetic field theretowith a magnet 18 so as to raise ionizing efficiency.

In the case where such efficient oxidization processing for P-typesuperconductor is used as the after-treatment process after thecrystallinity improvement and reduction processing, although thecritical temperature reaches the maximum value in a short time, thecritical current density continues to increase, depending on theprocessing time and it takes a processing time of ten and several hoursto reach the maximum value. This is considered to mean that theoxidization processing is affected by the diffusion behavior of oxygenand it takes a very long time for the processing to cover the entiresuperconductor thin film. Therefore, in order to obtain qualitysuperconductor thin film subjected to sufficient oxidization processing,it is considered preferable to perform the oxidization processingsimultaneously with the thin film deposition processing, thecrystallinity improvement and reduction processing or by interruptingthe thin film deposition processing and the crystallinity improvementand reduction processing, that is, while alternately repeating the thinfilm deposition process, the crystallinity improvement and reductionprocessing process and the oxidization processing process.

From the mere standpoint of performing the crystallinity improvement andreduction processing and the oxidization processing, it is the mostdesirable to perform the crystallinity improvement and reductionprocessing and the oxidization processing simultaneously with the thinfilm deposition, but since the processing temperature is limited to thedeposition substrate temperature, depending on the kind of thin filmdeposition process, a sufficient effect can not be otained, or on thecontrary, high energy oxygen may affect it adversely. The inventors ofthe present invention confirmed that in the case of performing thecrystallinity improvement and reduction processing and the oxidizationprocessing by interrupting the thin film deposition, that is,alternately repeating the crystallinity improvement and reductionprocessing process, the oxidization processing process and the thin filmdeposition process, an effect equal to or more than that can beobtained. In these thin film deposition processes, each constituentelement immediately after deposition is in the excited state, and inmany cases, it may take a time of serveral minutes to become stabilized,and particularly with respect to thin film of laminated structure, thedeposition process may be interrupted each time a thin film of oneperiod structure is deposited in order to achieve stability. If thecrystallinity improvement and reduction processing and the oxidizationprocessing are performed during the deposition interruption as describedabove, an ideal crystallinity improvement and reduction processing andthe oxidization processing are considered to be performed in a shorttime. The inventors of the present invention confirmed that byinterruption for each proper deposition time and performing thecrystallinity improvement and reduction processing and the oxidizationprocessing, a p-type superconductor thin film of an excellentcharacteristics may be obtained. They further confirmed that as saiddeposition time interval, it is effective to set it so that the filmthickness deposited during the time is more than 10 Å and less than 100Å.

Concrete Embodiment 1

By using magnesium oxide single crystal (100) surface as a substrate 11,by sputter-depositing target 13 formed of a sintered oxide hightemperature superconductor material by high frequency planar magnetronsputtering method in the atmosphere of Ar and O₂ mixture gas, a crystalY-Ba-Cu-O thin film was deposited on said substrate. In this case, thedeposition conditions were: a gas pressure of 0.4 Pa, sputteringelectric power of 160 W, sputtering time of 1 hour, thin film thicknessof 0.5 μm and a substrate temperature of 600° C. The respectivetemperature dependency of resistivity in the case where immediatelyafter the crystallinity improvement processing by X-ray irradiationusing Rh bulb and the film formation, oxygen gas is introduced into theformation tank and no oxidation processing as the after-treatment by thesame slow cooling process as used for sintered bodies is performed, inthe case where no crystallinity improvement processing by X-rayirradiation using Rh bulb is performed, and in the case whereimmediately after the crystallinity improvement processing by X-rayirradiation using Rh bulb and thin film formation, oxygen gas isintroduced into the formation tank, and the oxidization processing andthe after-treatment by the same slow cooling process as used forsintered bodies is performed are shown in FIGS. 2a, 2b and 2c.Furthermore, X-ray diffraction figures showing crystallinity for therespective cases are shown in FIGS. 3a, 3b and 3c. From FIG. 2, it isknown that as compared with the case where only the oxidizationprocessing is performed, in the case of using the crystallinityimprovement processing, further improvement of the superconductivecharacteristic is obtained. From FIG. 3, it is known that by thecrystallinity improvement processing, the orientation of thin film cface is improved so that the crystallinity is improved and by theoxidization processing, the face distance is reduced.

The inventors of the present invention tried, as the method forperforming the crystallinity improvement processing and the oxidizationprocessing, the following three kinds of methods: (1) a method ofperforming the same as the after-treatment of thin film deposition, (2)method of performing the same simultaneously with thin film deposition,(3) method of performing the same by interrupting thin film deposition,that is, method of alternately and periodically repeating the thin filmdeposition process, the crystallinity improvement processing process andthe oxidization processing process. In this case, as the oxidizationprocessing method, ECR oxygen plasma oxidization method shown in FIG. 1was used, and the processing conditions were microwave power of 200 Woxygen gas pressure of 8.5×10⁴ Torr, bias voltage of 50 V. In the case(3) of alternately and periodically repeating the thin film depositionprocess, the crystallinity improvement processing process and theoxidization processing process, the film thickness deposited in oneperiod, the crystallinity improvement processing time, and theoxidization processing time were respectively set at about 100 Å, 10minutes and 72 seconds. In each case of oxidization methods (1) and (2),the crystallinity improvement processing time by Hall ultraviolet rayirradiation was set at 2 hours, the oxidization processing temperaturewas set at 450° C., and the processing time was set at 1 hour. Theinventor of the present invention et al. confirmed that by using anycombination of the thin-film deposition process, the crystallinityimprovement processing process and the oxidization processing process ofcases (1), (2) and (3), a higher transition temperature and a largercritical current density may be obtained as compared with the case whereno crystallinity improvement processing and oxidization processing areperformed. The details of the variation of the superconductivecharacteristics due to the difference to the constituent elements ofcompound oxide thin film of this kind are not clear. The details of thevariation of the optimum conditions for the crystallinity improvementprocessing and oxidization processing are also not clear. However, it isclear that the crystallinity improvement processing and the oxidizationprocessing have a great effect on the superconductor characteristic andthe present invention establishes the crystallinity improvementprocessing process and the oxidization processing process for formationof a P-type superconductor thin film.

Preferred Embodiment 2

It was discovered that an excellent superconductor characteristic can beobtained by performing X-ray irradiation during formation of N-typesuperconductor thin film and holding thin film in the vacuum at atemperature within the range of 200° C. to 800° C. Furthermore, it wasdiscovered that the temperature range wherein the crystallinityimprovement and reduction by X-ray irradiation is performed the mosteffectively is a limited temperature range below the substratetemperature for thin film formation and above the normal temperature,and that the crystallinity improvement and reduction processing can beperformed the most efficiently and easily by performing the same for agiven time within this temperature range. The inventors of the presentinvention confirmed that this effect is also seen in the case ofperforming the crystallinity improvement and reduction processing byinterrupting thin film deposition, that is, alternately repeating thinfilm deposition process, and the crystallinity improvement and reductionprocessing process. Since the temperature for the crystallinityimprovement and reduction processing differs depending on thecomposition of thin film, and the surface condition thereof, selectionof an optimum temperature for each case is necessary, and the inventorsconfirmed that the optimum temperature is within a temperature range of600° C. to 900° C. Furthermore, with respect to the processing time, anecessary minimum value exists depending on the thin film composition,film thickness and the surface condition.

From the mere standpoint of performing a crystallinity improvement andreduction processing, it is the most desirably to conduct thisprocessing concurrently with the thin film deposition, but it wasconfirmed that since the processing temperature is limited to the thinfilm deposition temperature, depending on the kind of thin filmdeposition process, a sufficient effect may not be obtained in somecases, the inventors of the present invention confirmed that rather inthe case of conducting the crystallinity improvement and reductionprocessing by interrupting thin film deposition, that is, in the case ofalternately repeating the thin film deposition process, and thecrystallinity improvement and reduction processing process, an equal orlarger effect can be obtained.

Concrete Embodiment 2

By using a strontium titanic acid single crystal (100) face as substrate51 and by sputtering vapor-depositing target 53 formed of sintered oxidehigh temperature superconductor material by high frequency planarmagnetron sputtering method in the atmosphere of Ar and O₂ mixture gas,a crystal Nd-Ce-Cu-O thin film was deposited on said substrate. In thiscase, the processing conditions were: gas pressure of 0.3 Pa, sputteringelectric power of 160 W, sputtering time of 1 hour, thin film thicknessof 0.8 μm, and a substrate temperature of 650° C. By conducting thecrystallinity improvement and reduction processing by X-ray irradiationusing Rh bulb and by holding the thin film at 1100° C. and in the vacuumof 8×10⁻⁷ Torr, and excellent superconductive characteristic wasobtained. Respective temperature dependencies of resistivity in the casewhere no crystallinity improvement processing by X-ray irradiation usingRh bulb is effected and in the case where the crystallinity improvementand reduction processing by X-ray irradiation using Rh bulb is effectedare shown in FIGS. 6a and 6b. Furthermore, an X-ray diffraction figureshowing the crystallinity of FIG. 6b is shown in FIG. 7. From FIG. 6, itis understood that in the case where the crystallinity improvement andreduction processing is conducted, superconductive characteristics areobtained. Further from FIG. 7, it is understood that by thecrystallinity improvement and reduction processing, a thin film of highorientation is obtained.

The inventors of the present invention tried, as methods for conductingthe crystallinity improvement and reduction processing, the followingthree kinds of methods: (1) conducting said processing as theafter-treatment of thin film deposition, (2) conducting said processingconcurrently with thin film deposition, and (3) conducting saidprocessing by interrupting thin film deposition, that is, alternatelyand periodically repeating the thin film deposition process, and thecrystallinity improvement and reduction processing process. In the case(3) of alternately and periodically repeating the thin film depositionprocess, and the crystallinity improvement and reduction processingprocess, the film thickness deposited during one period and theprocessing time for crystallinity improvement and reduction were set atabout 100 Å and 10 minutes, respectively. The inventors of the presentinvention confirmed that by using any combination of the thin filmdeposition process, and the crystallinity improvement and reductionprocessing process classified in cases (1), (2) and (3), a hightransition temperature and a high critical current density can beobtained.

The details of the variation of the superconductor characteristics dueto the difference in the composition of these kinds of compound oxidesuperconductor thin film is not clear, nor are the details of thevariation of the optimum condition for the crystallinity improvement andreduction processing clear. However, there is no doubt that thecrystallinity improvement and reduction processing has a great influenceon the superconductor characteristics and the present inventionestablishes the crystallinity improvement and reduction processingprocess for new N-type superconductor thin films of Nd₂ CuO₄ crystalstructure.

Preferred Embodiment 3

FIG. 8 is a basic constitution sectional drawing for a thin filmsuperconductor according to an preferred embodiment of the presentinvention which is characterized by irradiating photons at an incidentangle below the total reflection angle. In the same formation tank,there are provided a vapor deposition source 81 (here, as an example,high frequency magnetron sputtering device) for thin film deposition, adevice 82 (here, as an example, Rh X-ray bulb) for irradiating photonshaving energy larger than ultraviolet rays onto deposited thin filmsurface at an incident angle below the total reflection angle for makinga surface oxygen deficiency and an oxidization processing device 83(here, as an example, an ECR oxygen plasma generating device).Superconductor thin film 84 is formed on a single crystal substrate 85.It was confirmed that though the thin film deposition process and thephoton irradiation process are alternately repeated, when the photonirradiation interval is so set that the thin film thickness depositedduring said interval is more than 10 Å and less than 100 Å, it iseffective for pinning fluxoid quantum lines. This relects that thecoherence length of high temperature superconductor is short and showsthe oxygen deficiency is smaller in the free energy as compared with thesurrounding and becomes an effective pinning center for the fluxoidquantum lines in the critical state.

In other words, when oxygen deficiencies are distributed, as shown inFIG. 9, in the film thickness direction at an interval of severalangstroms to a hundred angstroms, it is considered possible to pinfluxoid quantum lines over the entire film thickness.

Preferred Embodiment 4

The magnetization curves measured at a constant temperature for asuperconductor thin film sample subjected to a crystallization reductionprocessing and that not subjected to said processing are shown in FIG.10a. A SQUID magnetic fluxmeter was used and an external magnetic fieldwas applied in parallel to the c axis (that is, in the directionperpendicular to the film surface). The change in the magnetization bythe magnetic field of the sample not subjected to the processing is theusually observed change of a superconductor of the second order. In thesample subjected to the crystallization reduction processing, as theexternal magnetic field is increased, the diamagnetism increasesapproximately linearly and at a certain magnetic field, it abruptlydrops to several tenths. The diamagnetism once decreased can not berecovered even when the external magnetic field is returned to zero. Thediamagnetism value under the constant external magnetic field almoststays unreduced for the time-dependent change. These results areconsidered to be ascribed to the fact that the intensity distribution ofthe flux pinning force in the superconductor thin film is homogeneous.That is, when an external magnetic field is applied to "a superconductorthin film subjected to crystallization reduction processing" cooledunder zero magnetic field, it behaves as a perfectly diamagnetic body upto a certain external magnetic field and shows a large diamagnetism.However, once the intrusion pressure into the interior of thesuperconductor by the external magnetic field exceeds a certain fluxpinning force, the magnetic flux intrudes into the inside and thediamagnetism value decreases at once. When the "superconductor thin filmsubjected crystallization reduction processing" is once intruded on byflux, the diamagneticism value will not be recovered even when theexternal magnetic field is returned to zero. Such a singular magneticbehavior was fist discovered by the inventors of the present invention.

Regarding p-type superconductors, since a strong superconductor isprepared by irradiation of photons having energy larger than ultravioletrays accompanied by an oxidization processing, and a weak superconductoris prepared by an oxidization processing only, when an oxidizationprocessing is conducted over the entire surface after photon irradiationpatterned by using photo-lithography technique, a bit patter as shown,for example, in FIG. 10b can be constituted and the manufacture of asuperconductive magnetic memory utilizing the above-describedcharacteristic. The read-out of the superconductive magnetic memory canbe made easily by utilizing the fact that the magnetic field immediatelyabove the film surface varies greatly with magnetic detectors such asmagnetic heads and Hall element heads.

Furthermore, there is a method for writing into the superconductivemagnetic memory which is so arranged that a perfectly diamagnetic stateunder a constant magnetic field is made to correspond to 1 and aminutely diamagnetic state intruded on by flux is made to correspond to0, and after setting all memory bits at 0 in advance, 0 bits are made byintroducing flux into a portion of memory bits through application ofpulse magnetic field. Here, changing from 0 to 1 is effected by heatingabove the transition temperature with laser pulses, etc. and aftercooling in the zero magnetic field, applying the external magnetic fieldagain. The inventors of the present invention confirmed that thesesuperconductive magnetic memories are small in the time-dependent changeand very stable from the time viewpoint. The effect of thecrystallization reduction processing on this superconductor thin film isthe same whether it is a p-type superconductor or n-type superconductor.

Concrete Embodiment 4

By using a formation device shown in FIG. 11, using magnesium oxidesingle crystal (100) face as the substrate 111 and sputtervapor-depositing the target 112 formed of oxide high temperaturesuperconductor material sintered by high frequency planar magnetronsputtering method in the atmosphere of Ar and O₂ mixture gas, a crystaly-Ba-Cu-O thin film 115 was deposited on the substrate 111 heated by asubstrate heating 16. In this case, the processing condition was a gaspressure of 0.4 Pa, sputtering electric power of 160 W, sputtering timeof 1 hour, thin film thickness of 0.5 μm and substrate temperature of600° C. The crystallinity improvement processing was conducted by X-rayirradiation using an Rh bulb, and the oxidization processing wasconducted by introducing oxygen gas into the formation tank 112immediately after formation of superconductor thin film 115, as is clearfrom the magnetic field characteristic curve in FIG. 10a, in thesuperconductor thin film not subjected to the crystallization reductionprocessing in the present invention, diamagnetism slowly changes and theflux intrudes slowly. However, in the superconductor thin film subjectedto said processing, at T=48 K., magnetization increases almost linearlyup to H=200 Oe, and thereafter shows maximum magnetization 1000 Oemu/ccat H=400 Oe. When magnetic field is further strengthened, flux intrudesinto the superconductor thin film, resulting in the drop ofmagnetization value to about one to several tenths. When sample of about1 mm outside diameter is measured with Hall element for low temperatureuse, at T=48 K., H=4000 e at diamagnetic state (M=1000 Oemu/cc) andH=200 Oe at flux intrusion state, and the diamagnetic state has a muchlarger surface magnetic field as compared with the flux intrusion stateand this difference can be detected easily.

Preferred Embodiment 5

A superconductor thin film is formed on the substrate by the sputteringmethod, and by heat treating in the oxygen atmosphere or by irradiationoxygen ions or neutral oxygen atoms in the excited state,superconductivity is generated. Furthermore, by irradiating X-rays orultraviolet rays into a part of said superconductor thin film, a weakcoupling portion is formed. However, the inventors of the presentinvention confirmed that this weak coupled potion shows any one ofordinary conductor, semiconductor or insulator, depending on theirradiation amount of X-rays or ultraviolet rays. In this case, whenthere are two superconductor electrodes, that is, it is a two-terminalelement, the weak junction portion becomes a Tunnel type Josephsonelement. It is confirmed by the inventors of the present invention thatthis thin film formation is not limited to the physical vapor phasegrowth method, chemical vapor phase growth method, for example,atmospheric or decompression chemical vapor phase growth method, plasmachemical vapor phase growth method, photo-chemical vapor phase growthmethod are effective when the composition is conformed. It was furtherfound by them that when the superconductivity obtained by oxygenprocessing these thin films is controlled by irradiation of X-rays orultraviolet rays, these superconductor thin films show the property ofordinary conductors, semiconductors or insulators. Said X-ray orultraviolet ray irradiation processing is conducted by using ordinaryX-ray bulb of W, Mo, Rh, Cu, Fe, Co, Cr, Al, Mg, ete. or ultraviolet raysource of H, he, Ne, etc., and by irradiating X-rays or ultraviolet raysonto superconductor thin films. This effect is considered to be resultfrom the fact that the thin film superconductor optimized to obtainsuperconductivity by the oxidization processing is reduced byirradiation of X-ray or ultraviolet ray, the optimum condition of oxygenfor superconductivity is broken by control of the oxygen amountcontained in the crystal, and the superconductor characteristics of theportion irradiated by X-rays or ultraviolet rays vanished.

Unlike the utilization of ion beams or electron beams, the utilizationof X-rays or ultraviolet rays makes the homogeneous processing possiblewithout causing change of ratio among metallic atoms, local greattemperature rise and crystallinity deterioration of thin film, andtherefore, the present invention has an advantage of causing no seriousdamage to the on thin film. Moreover, X-rays and ultraviolet rays areshort in wavelength, minutely control area of superconductor thin filmto be processed, and make possible the reduction processing of preciseand minute cuprooxide thin film. Thereby, the preparation of devicessuch as Josephson element requiring sub-micron processing becomes easyby irradiation of X-rays or ultraviolet rays.

Preferred Embodiment 6

A preferred embodiment of the present invention will be described withreference to the accompanying drawings.

In FIG. 12, a superconductor thin film 122 is formed on a substrate 121,for example, by the sputtering method. Furthermore, by conducting heattreatment in the oxygen atmosphere in accordance with the needs, orirradiating oxygen ions or neutral oxygen atoms in the excited state,superconductivity is obtained. Further, by irradiating photons havingenergy larger than ultraviolet ray on the thin film 122 surface, andthereafter by selectively conducting oxidization processing, twoelectrode portions 123 separated by a gap portion 124 are formed. As themethod for said oxidization processing, irradiation of at least oxygenions generated by discharging of an oxygen containing gas or neutraloxygen atoms in the excited state, or treatment in the ozone containinggas were found to be effective and easy. These two oxidization processesboth do not require heating samples, and besides, the former has anadvantage of completing the processing in a comparatively short time,while the latter has an advantage that since organic materialsrepresented by resist are little impaired, an ordinary resist film maybe used for the mask in the oxidization processing. It was furtherconfirmed by the inventors of the present invention that the electrodeportions 123 have superconductivity in the thin film 122, and portionincluding the gap portion 124 other than electrode portions 123 is of anordinary conductor, semiconductor, or insulator, because no photons areirradiated or no oxidization processing is conducted thereon. In thiscase, the gap portion 124 becomes a weak coupling or a tunnel junction,and if it is a two-terminal element, it becomes a Josephson element.

The inventors of the present invention found that as described above, byirradiating X-rays, ultraviolet rays or photons having larger energythan ultraviolet rays onto a superconductor thin film, thissuperconductor thin film can be controlled so as to show the property ofan ordinary conductor, semiconductor or insulator. As photons ofwavelength shorter than ultraviolet rays, radiation light from ordinaryX-ray bulbs such as W, Mo, R, Cu, Fe, Co, Ct, Al, Mg, Zr, an ultravioletray source such as H, He, Ne, etc. or mercury lamp, or γ rays radiatingfrom radiative elements by γ disintegration may be used. FIG. 13 showsan example of the result of the basic experiment for confirmation ofthis characteristic, and taking Bi-Sr-Ca-Cu-O superconductor thin film,there are shown the measured temperature change of resistivity conductedin the three conditions of immediately after preparation of film (131),after ultraviolet ray irradiation for three hours (132), and after theoxidization processing conducted, thereafter. Though superconductivitywas shown at about 45K immediately after preparation of thin film (131),the resistivity at the room temperature increased about 8 times afterirradiation so as to show now superconductivity (132), but thecharacteristic immediately after preparation was almost recovered by theoxidization processing. This result shows that irradiation reduces thethin film and causes superconductivity vanish but the characteristic canbe reversibly recovered by the oxidization processing.

Concrete Embodiment 6

By using (100) face MgO single crystal as as substrate an by sputtervapor-depositing YBa₂ Cu₄.6 O_(m) target sintered by high frequencyplanar magnetron sputtering method in the atmosphere of Ar and oxygenmixture gas, crystal YBa₂ Cu₃ O₇ thin film was deposited on saidsubstrate.

In this case, the processing condition was a gas pressure of 0.5 Pa,sputtering electric power of 150 W, sputtering time of 20 minutes, thinfilm thickness of 0.2 μm, and a substrate temperature of 700° C. Thethin film thus obained showed superconductivity and the transitiontemperature was 90K.

Although the thickness of thin film 122 was 0.2 μm in the presentinvention, it was confirmed that superconductivity is generated in thecase of a thin film of less than 0.1 μm thickness or in the case of thinfilm of more than 10 μm thickness.

Furthermore, by irradiating ultraviolet rays on this superconductivethin film 122 for about 3 hours with a low tension mercury lamp, saidfilm 122 was reduced so as to cause superconductivity to vanish. And,after forming a mask pattern on the superconductor thin film 142 byusing resist film 143 as shown in FIG. 14, said superconductor thin film143 was left for several hours in the oxygen atmosphere containingseveral percent of ozone by utilizing an ozone generator, and theoxidization processing was conducted on the portion not covered byresist film 143 in the surface of superconductor thin film 142 so as toform electrode portions there.

Finally, the resist film 143 was removed with organic solvent, measuringterminals were bonded to two electrode portions with conductive paste, aalternating voltage of 85 Hz was applied to these terminals, and throughthese terminals, the relationship with the current flowing through thegap portion was observed. As a result, the observed current-voltagecharacteristic had a non-linearity characteristic of Josephson elements.Furthermore, when microwaves of 20 Ghz were irradiated on this gapportion 145, a step of voltage was observed on the current-voltagecharacteristic curve, and from the relationship between the position ofthis step and microwave frequency, this step was found to be a so-calledShappiro step characteristic of the Josephson junction. From theseresults, it was found that a SNS type (superconductor-normalconductor-superconductor) Josephson junction was formed in the trialmanufacture element.

By this method, it was possible to form a Josephson element of goodcontrollability and weak coupling. In this case, as resist, PMMA forelectron beams which can be developed with organic solvent only ornegative resist and the like are suitable. Usually, many of theseresists are changed in quality when heated above 200° C., and can notdisplay the function as a resist, but the oxidization method accordingto the present invention is effective in that it neither hurts theresist nor raises the substrate temperature.

Preferred Embodiment 7

By forming superconductor thin film on the substrate by the sputteringmethod, and by conducting heat-treatment in the oxygen atmosphere orirradiating oxygen ions or neutral oxygen atoms in the excited state asneeded, superconductivity is obtained. Next, photons having an energylarger than ultraviolet rays are selectively irradiated on two electrodeportions separated by a minute gap portion on the thin film surface. Andsimultaneously with the irradiation or thereafter, an oxidizationprocessing is conducted on the thin film surface. It was confirmed thatthe portion not subjected to photon irraadiation by this method isinferior in superconductive characteristics as compared with theelectrode portions irradiated with photons, and by controlling thecondition of the irradiation and the oxidization processing, a weakcoupling type Josephson junction is made. According to the manufacturingmethod of the present invention, since no selective oxidizationprocessing is required but the oxidization processing all over thesample is possible, it is not necessary to use masks of such as resistfilm which is easily eroded by oxygen plasma in the oxidizationprocessing. Therefore, there is an advantage that the oxidizationprocessing can be made by using oxygen plasma capable of conducting saidprocessing in a short time. Furthermore, it is not always necessary touse a resist for a mask and by the usual technique of photo-lithography,for example, by bringing the usual photomask made by forming patternedthin film of chromium a glass plate into contact with superconductorthin film and by irradiating through this mask, it is possible toirradiate photons selectively on the electrode portions.

Preferred Embodiment 8

A superconductor thin film is formed on a substrate by the sputteringmethod and by conducting heat treatment or by irradiating oxygen ions orneutral oxygen atoms in the excited state as needed, super conductivityis obtained. Next, photons having an energy larger than ultraviolet raysare selectively irradiated on the two electrode portions separated by aminute gap portion on the thin film. And simultaneously with saidirradiation or thereafter, an oxidization processing is conductedselectively on said electrode portions. It was confirmed that the gapportion not subjected photon irradiation by this method is inferior insuperconductor characteristics as compared with the electrode portionsirradiated with photons, and by controlling the condition of irradiationand oxidization processing, it becomes a weak coupling type Josephsonjunction. Since portions other than the electrode portions are notsubjected to photon irradiation or oxidization processing according tothe present manufacturaing method, in the case of integrating otherfunctional elements on the same substrate, the present manufacturingmethod has an advantage of not adversely affecting these functionalelements.

Since the irradiation of photons having energy larger than ultravioletrays finely controls, because of the short wavelength of photons, thearea of superconductor thin film to be processed and with fineprecision, allows the reduction processing of superconductor thin films,the preparation of devices such as Josephson devices requiringsub-micron processing is easily made by irradiation of photons.

Preferred Embodiment 9

A preferred embodiment of the present invention will be described withreference to the accompanying drawings.

FIG. 15 is a basic constitution sectional view of a superconductor thinfilm transistor according to a preferred embodiment of the presentinvention, and FIG. 16 is a sectional process drawing showing themanufacturing method for the superconductor thin film transistoraccording to the present invention. In FIG. 15, on the substrate 151,there are formed a superconductive source area 152 and a superconductivedrain area 153, each constituted of material made of oxide thin filmrepresented by A-B-Cu-O, and improved in crystallinity over said channellayer by irradiation of photons having shorter wavelength thanultraviolet ray. and on the same substrate, there is formed so as tocontact these areas a channel layer thin film 154 made of oxide thinfilm composed of the same constituent elements. Here, A designates atleast one kind of element among Bi, Tl, Pb, Y and lanthanide serieselements (elements of atomic number 57 through 71, however excludingthose of Nos. 58, 59 and 61), and B designates at least one kind ofelements among elements of 2a group. On the channel layer 154, there arefurther formed a gate insulating film 155 and on top thereof a gateelectrode 156. The superconductor transistor according to the presentinvention is an element of extremely short wavelength of about 0.1 μm,and for manufacture of this element, the following method is veryeffective.

The inventors of the present invention discovered that by irradiation ofphotons having energy larger than ultraviolet rays, the crystallinity ofsuch a oxide superconductor thin film is improved, and further confirmedthe rise of the critical temperature and improvement of critical currentdensity by combining the oxidization processing therewith. By this andby using the gate electrode as the mask for photons, a self-alignmentprocess applicable to a minute gate electrode structure was invented.

With reference to FIG. 16, a preferred embodiment of the manufacturingmethod for the superconductor transistor according to the presentinvention will be described. By using oxide magnesium single crystal(100) face as the substrate 161, and by sputter vapor-depositing targetformed of high temperature superconductor sintered by the high frequencyplanar magnetron sputtering method in the atmosphere of Ar and oxygenmixture gas, oxide thin film 168 was deposited on said substrate ascrystal Y-Ba-Cu-O thin film (FIG. 16a). In this case, the processingconditions were: a gas pressure of 0.4 Pa, sputtering electric power of100 W, sputtering time of 10 minutes, thin film film thickness of 50 nm,and substrate temperature of 600° C. Next, all over the surface of theoxide thin film 168, gate insulation film 165 and gate electrode 166were formed by sputtering vapor deposition. As a gate insulation film, azirconium film of 20 nm thickness was used and as gate electrode 166, Ptof 20 nm thickness was used, This was etched by the photoprocess and Arion etching technique until the surface of the oxide thin film 168 isexposed with only the gate eletrode portion being left (FIG. 16b). Thegate electrode length was set at 0.2 μm and the width thereof was set at5 mm. Thereafter, the surface of the oxide thin film 168 is subjected tothe oxidization processing after or during irradiation of photons 167having energy larger than ultraviolet ray, for example, X-ray. As aresult, the gate electrode 166 becoming the mask for photons 167, theoxide thin film 168 under the gate electrode 166 becomes a channel layerthin film 164 showing semiconductive resistance or a weaksuperconductive characteristic. Also, the oxide thin film 168 having nogate electrode thereabove is improved of in crystallinity by irradiationof photons 167 and becomes a superconductor thin film of high criticaltemperature and large critical current density and forms respectivelythe source area 162 and drain area 163, and thus the element iscompleted. For X-ray irradiation, the X-ray source using Rh bulb isused, and as the method of the oxidization processing, the heattreatment at 200° C. to 800° C. in the oxygen atmosphere or irradiationof oxygen plasma are effective. An example of the processing conditionis microwave power of 200 W, oxygen gas pressure of 8.5×10 H-4 Y Torr.,and bias voltage of 50 V. It was confirmed that the prepared elementobtained a critical current of 10 μA at 4.2K and by applying 20 Vvoltage between gate and source, said current dropped to 5 μA and by theelectric field, the control of superconductive current is possible.

It is to be noted here that although magnesium oxide was used forsubstrate in the present embodiment, the present invention is notlimited to this, and any material may be applicable so long as it has asimilar function. Furthermore, although Y-Ba-Cu-O was used as thematerial for oxide thin film 168, it is not limited to the material butany material may be used so long as it is such material as described inthe foregoing embodiment. Moreover, zirconium oxide was used formaterial of gate insulating film, it is not limited to this material butany material having similar function may be used. Furthermore, althoughPt was used for gate electrode material, it is not limited to thismaterial but any material having similar function may be applicable.

EFFECT OF THE INVENTION

Since a process for insuring the reliability and long range stability ofthe element using high temperature superconductor is provided by thepresent invention, the present invention has an extremely great valuefrom the industrial viewpoint. Since the superconductor thin film ishomogeneous and of thin film single crystal, as compared withconventional sintered body, it becomes possible to realizesuperconductor elements of extremely high accuracy by the presentinvention. It has a great feature in providing an efficient and easyprocess for crystallinity improvement processing and oxidizationprocessing for a P-type superconductor. Also, by the present invention,a process for insuring the reliability and long range stability of theelements using an N-type thin film superconductor is provided. It has anexcellent feature in providing a process for crystallinity improvementprocessing and oxidization processing for an N-type superconductor.

In the foregoing preferred embodiments, high temperature superconductorincluding cupro-oxide was described, the effect of the present inventionis effective with any high temperature superconductor wherein oxygendeficiency is generated by photon irradiation. Therefore, the presentinvention is effective even for materials not containing copper, forexample, BKB (Ba-K-Bi-O) series material. Since in a sulphur compoundwherein oxygen was replaced by sulphur, sulphur efficiency is generatedby photon irradiation, the effect of the present invention is similarlyexpected in Chevrel compounds.

From the fact that the current density in the superconductive state issmall and the time-dependent logarithmic decrease of current density isremarkable, it has been thus far considered difficult to realize asuperconductor device of high reliability. However, by dint of thepresent invention, it has become possible to control on the atomic scaleand introduce an effective pinning center for fluxoid quantum linescomposed of oxygen deficiency and it was made possible to improve thecritical current density and at the same time to increase the activationenergy of flux creep. This has provided a base for realizingsuperconductor devices of high stability and reliability with a hightemperature superconductor.

By the crystallization and reduction processing through irradiation ofphotons having energy larger than ultraviolet rays effected duringformation of thin film of the present invention or thereafter, it ispossible to realize a high temperature superconductor thin film ofstrengthened flux pinning force and superconductive magnetic memory ofexcellent memory retaining characteristic using the superconductor thusprepared.

The manufacturing method for thin film superconductor elements accordingto the present invention has a great feature in that photons havingenergy larger than ultraviolet ray are irradiated partially on thehomogeneous superconductor formed as a thin film. The reductionprocessing for a cupro-oxide thin film by photon irradiation does not atemperature rise of the thin film, has a good controllability and issimple to process. Therefore, a homogeneous superconductor element witha high accuracy weak coupling portion is easily realized by the presentinvention.

The manufacturing method for Josephson elements according to the presentinvention is greatly characterized by the fact that photons havingenergy larger than ultraviolet rays are irradiated on a homogeneoussuperconductor formed as a thin film and the oxidization processing isconducted thereon. The processing of the superconductor thin film doesnot cause a temperature rise of the thin film, is excellent incontrollability and is simple to handle. Therefore, homogeneousJosephson elements having a high accuracy weak coupling portion can beeasily realized. Furthermore, the integration with devices, for example,of Si or GaAs becomes possible. The manufacturing method for Josephsonelements according to the present invention may be practicallyapplicable to the manufacture of various superconductive devices such asSQUID. Moreover, the present invention provides a superconductortransistor using high temperature superconductor thin film of extremelygood characteristics. By practicing the present invention, a processwhich insures the reliability and long range stability of thesuperconductor transistor using a high temperature superconductor andfacilitates the manufacture thereof is provided, by the presentinvention. A superconductor transistor of very high accuracy can berealized.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of the present invention. Accordingly, it isnot intended that the scope of the claims appended hereto be limited tothe description as set forth herein, but rather that the claims beconstrued as encompassing all the features of patentable novelty thatreside in the present invention, including all features that would betreated as equivalents thereof by those skilled in the art to which thepresent invention pertains.

What is claimed is:
 1. A method for manufacturing a thin film P-typeoxide superconductor element which comprises:providing an oxidesuperconductor thin film in which a critical current density obtainedfrom a magnitude of diamagnetization measured at 48 K. in an outermagnetic field of 150 Oe is more than 3 mill.A/cm² and the criticalcurrent density obtained from a transport current measured at 48 K. in anull magnetic field is more than 3 mill.A/cm² by forming an oxidesuperconductor thin film on a substrate and irradiating said oxidesuperconductor thin film with photons having energies larger than thatof ultraviolet rays and forming a weak coupling portion on said thinfilm oxide superconductor by irradiating photons having energies largerthan that of ultraviolet rays onto a minute portion of said thin film toform a structure wherein said thin film oxide superconductor is dividedinto at least two regions by said weak coupling portion.
 2. A method formanufacturing a Josephson device which comprises:providing a P-typeoxide superconductor thin film in which a critical current densityobtained froom a magnitude of diamagnetization measured at 48 K. in anouter magnetic field of 150 Oe is more than 3 mill.A/cm² and a criticalcurrent density obtained from the transport current measured at 48 K. ina null magnetic field is more than 3 mill.A/cm² by forming a thin filmoxide superconductor on a substrate and irradiating said thin film withphotons having energies larger than that of ultraviolet rays, andselectively oxidizing two regions on said thin film oxide superconductorwhich are separated by a minute gap portion to form a junction portionin said oxide superconductor thin film.
 3. The method for manufacturinga Josephson device in accordance with claim 2 which comprises using amask pattern for forming said junction portion, and employing as saidmask pattern an electron beam resist made of an acrylic resin or styreneresin or a negative resist for optolithography.
 4. The method formanufacturing a Josephson device according to claim 2 wherein saidoxidating step is performed by irradiation of oxygen ions orquasi-stabilized oxygen atoms is an excited state.
 5. The method formanufacturing a Josephson device in accordance with claim 2 wherein saidoxidizing step is performed using a gas containing ozone.
 6. A methodfor manufacturing a Josephson device which comprises selectivelyirradiating photons having energies larger than that of ultraviolet raysonly onto two regions which are separated by a minute gap portion ofP-type thin film oxide superconductor in which a critical currentdensity obtained froim a magnitude of diamagnetization measured at 48 K.in an outer magnetic field of 150 Oe is more than 3 mill.A/cm² and acritical current density obtained from the transport current measured at48 K. in a null magnetic field is more than 3 mill.A/cm² by forming athin film oxide superconductor on a substrate and irradiating said thinfilm with photons having energies larger than that of ultraviolet rays,and oxidizing a surface of the oxide superconductor thin film duringsaid irradiating step or after said irradiating.
 7. A method formanufacturing an oxide superconductor transistor, the oxidesuperconductor transistor including a gate electrode formed on a surfaceof a channel region with a gate insulating film interposed therebetween,and a superconductor source region and a superconductor drain regioneach comprising a strong superconductor in which a critical currentdensity obtained from a magnitude of diamagnetization measured at 48 K.in an outer magnetic field of 150 Oe is more than 3 mill.A/cm² and thecritical current density obtained from a transport current measured at48 K. in a null magnetic field is more than 3 mill.A/cm², said methodcomprising:forming an oxide superconductor thin film on a substrate,forming a gate isulating film and a gate electrode on a surface portionof said oxide superconductor thin film corresponding to said channelregion, and irradiating photons having energies larger than that ofultraviolet rays onto said oxide superconductor thin film using saidgate electrode as a mask and oxidizing said superconductor thin filmduring or after said irradiating step, thereby improving crystallinityin the superconductor source area and superconductor drain area andconcurrently forming the channel layer.
 8. The method for manufacturingan oxide superconductor transistor in accordance with claim 7,comprising forming the gate electrode of a metal or a silicide.
 9. Amethod for manufacturing an oxide superconductor magnetic memory whichis capable of distinguishing memory states responsive to an absolutevalue of magnetization and which includes at least one strongsuperconductor in which the critical current density obtained from amagnitude of diamagnetization measured at 48 K. in an outer magneticfield of 150 Oe is more than 3 mill.A/cm² and a critical current densityobtained from the transport current measured at 48 K in a null magneticfield is more than 3 mill.A/cm² and at least one weak superconductorhaving a critical current density smaller than that of said strongsuperconductor, said method comprising preparing said strongsuperconductor by irradiating a thin film oxide superconductor withphotons having energies larger than that of ultraviolet rays accompaniedby oxidation, and preparing said weak superconductor by only irradiatingthe thin film oxide superconductor with photons having energies largerthat that of ultraviolet rays.